1. Field of Invention
This invention relates generally to a plastic anchor which when installed in a hole drilled in a wall is adapted to receive a threaded fastener that serves to hold a fixture or other object against the wall, and more particularly to a self-drilling anchor of this type which dispenses with the need for any tool other than a screwdriver to install the anchor.
2. Status of The Prior Art
In order to mount brackets, fixtures and other objects provided with mounting holes against the outer surface of a wall formed of plasterboard, sheetrock, fiberboard or any other material employed in hollow wall construction, the common practice is to use a hollow anchor for this purpose. The conventional hollow anchor is formed of metal such as zinc or of plastic such as PVC, which when hammered into and wedged within a hole pre-drilled in the wall, is then adapted to receive a threaded fastener or mounting screw that goes through the mounting hole in the object and turns into the hollow of the anchor.
The required length of the mounting screw depends on the thickness of the object to be mounted. Thus if the object to be mounted is a thin metal bracket, a screw of smaller length is required than if the object is a thick board, for this dictates a longer screw.
The installation of a conventional anchor entails three distinct operations, the first and most critical step being the drilling of a hole in the wall, for the diameter of this hole must be appropriate to that of the anchor. Should an oversize hole be drilled, then in the next step when the anchor is hammered into the hole, the anchor will not wedge firmly therein but will be somewhat loose. As a consequence, it will not be possible to carry out the third step.
In the third step, one must turn a threaded fastener into the hollow of the anchor, and in doing so tap the anchor. But if the anchor is somewhat loose in the wall hole, this will cause the anchor to turn as the fastener is turned, and the fastener will therefore not be able to thread its way into the anchor.
There is also a possible fourth step that must be taken into account; for should a need arise to remove an object mounted on the wall or replace it with another object, then one must first unscrew the fastener from the anchor lodged in the wall. However, if when an unscrewing torque is applied to the fastener, this torque causes the anchor as well as the fastener to turn, then it will not be possible to withdraw the fastener from the anchor. The need therefore exists for an anchor which, after being lodged in a hole drilled in the wall, will resist being turned counterclockwise and thereby make it possible to unscrew a threaded fastener from the installed anchor.
The installation of a conventional hollow anchor requires three tools: a drill provided with a drill bit having a diameter appropriate to that of the anchor; a hammer to drive the anchor in the hole drilled in the wall; and a screwdriver to turn the threaded fastener into the anchor or to later remove it from the anchor.
Though a typical householder or other non-professional possesses a hammer and a screwdriver, he may lack a drill. But even if he has a drill, he may not have a drill bit of the appropriate diameter for the anchor to be installed. Yet there are distinct advantages to be gained by using hollow anchors to mount objects against a wall, rather than screws, nails or other fasteners which require no drilling but which have relatively poor holding power, particularly in walls made of friable materials. Nevertheless, the need for drilling has to a significant degree heretofore discouraged the use of hollow anchors by non-professionals.
Though hollow wall anchors are used on a large scale by professional installers, the need to drill holes and then hammer the anchors into the holes is a practical drawback, for these operations are time consuming and hence impose distinct limits on the number of anchors that can be installed in a given period. And if anchors of different sizes must be installed, then the installer must change the drill bit when switching from one anchor size to another, and this, too, consumes time and reduces the productivity of the installer.
To overcome these drawbacks, the 1986 patent to Ernst et al., U.S. Pat. No. 4,601,625, discloses an anchor for drywall that has an externally-threaded cylindrical section adapted to receive a fastener, and an unthreaded drilling section. This anchor is installable using only a hand-powered screwdriver for this purpose, thereby dispensing with the need for a drill and a hammer. At the upper end of the anchor is a low profile flange or head which allows the anchor to be installed flush with the drywall surface.
The Ernst et al. patent discloses an embodiment of an anchor molded of zinc, and another embodiment formed of plastic material. Both embodiments include a drill section that can be deflected laterally upon insertion of a mounting screw therethrough. This makes it possible to use a single mounting screw of moderate length in conjunction with the anchor for attaching objects of various thicknesses to the wall.
In Ernst et al., the drill section is formed by a single flat blade whose free end is notched to define a central spike and a pair of peripheral spikes on either side of the central spike, the function of the central spike, which extends beyond the peripheral spikes and therefore is the first to penetrate the surface of the drywall, is to maintain the location of the rotating drill section as a hole is drilled by the peripheral spikes. In practice, when a single blade is used to bore a hole in friable material such as drywall, the resultant drilled hole may not be perfectly round, for when this material encounters the central or guide spike, it tends to make the blade whip, as a consequence of which the drilling is shifted off center and the bored hole is oversize or elliptical.
My above-identified copending application also discloses a self-drilling anchor having an externally-threaded shank section that terminates in a drilling section having a guide tip. In contradistinction to the cylindrical, externally-threaded section in Ernst et al., the shank section of my anchor has a root which tapers toward the drill section and a threading surrounding the root formed by a series of convolutions whose crests are of essentially the same diameter along substantially the full length of the shank section. The advantage of this arrangement is that as the anchor is screwed into a wall of friable material, the pulverized material is then packed by the threading into a relatively dense mass which surrounds the shank to enhance its resistance to withdrawal from the wall.
And while the anchor disclosed in my copending application also has a flange-like head of low profile similar to that of Ernest et al. so that the installed anchor will lie flush against the surface of the wall, in both cases these self-drilling anchors when installed by being turned into a wall do not offer adequate resistance to being turned out. Yet resistance to being turned may be necessary; for when an occasion arises requiring withdrawal of the mounting screw from the anchor, in turning out the screw with a screwdriver, this action will also cause the anchor to turn out with the screw; hence the screw cannot be withdrawn from the anchor.